1. Field of the Invention
The present invention relates to a surface acoustic wave resonator unit suitable to form a high-frequency oscillation circuit and an apparatus that exhibit excellent stability.
2. Description of Related Art
A technique for machining small articles, such as integrated circuits, causes fine electrodes to be formed on the surface of a piezoelectric member, thus enabling surface acoustic waves (SAW) to be electrically driven or detected. By using this technique, high frequency waves from about 100 MHz to the GHz level have been stably obtained. A surface acoustic wave device using the surface acoustic waves has been used to form a high-frequency filter (a surface acoustic wave filter) or a surface acoustic wave resonator unit (a SAW resonator unit) for forming an oscillation circuit.
FIG. 27 shows a known surface acoustic wave device. The device 90 has a surface acoustic wave resonator 92 that is, by an adhesive agent 93, secured to a supporting metal portion 91 and sealed in a case 96, by resistance welding or the like, performed in a nitrogen atmosphere. Leads 94 for establishing the electrical connection with the surface acoustic wave resonator 92, penetrate an insulating portion in the metal portion 91, which is a sealing glass 97. The leads 94 are electrically connected to electrodes disposed on the surface acoustic wave device by bonding wires 95.
The known surface acoustic wave device 90 shown in FIG. 28 is a so-called entire-surface adhesive type surface acoustic wave device that has: a base 101 made of ceramic or the like; and a surface acoustic wave resonator 92 attached to the base 101 by an adhesive agent 93. The base 101 has electrodes metallized thereon for establishing the electrical connection with the surface acoustic wave resonator 92 so that the electrodes on the base 101 and the electrodes on the surface acoustic wave resonator 92 are electrically connected to one another by bonding wires 95 similarly to the foregoing structure. Furthermore, a cap 102 is, by an adhesive agent or the like, attached above the base 101 in an atmosphere of nitrogen. The cap 102 is sometimes attached by brazing, welding or the like.
FIG. 29 shows a surface acoustic wave resonator 110 to be accommodated in the foregoing surface acoustic wave device. The surface acoustic wave resonator 110 is formed by using a piezoelectric member 111, such as a quartz crystal member. The piezoelectric member 111 is obtained by cutting a flat piezoelectric substrate to a predetermined size and dimension, the piezoelectric member 111 being usually formed into a rectangular shape as later described by cutting because an area for disposing a reflector can be obtained, a satisfactory mass productivity can be realized, and machining can easily be performed. An interdigital transducer (IDT) 112 is formed in a substantially central portion of either side (the main surface) of the piezoelectric member 111 by using a thin film electrodes made of aluminum material or the like. Furthermore, reflectors 113, each of which is made of a thin film of aluminum material or the like similar to the electrodes, are disposed on the two sides of the IDT 112 in the lengthwise direction of the same, that is, on the two sides of the longer sides of the piezoelectric member 111. Along the lengthwise directional edges of the piezoelectric member 111, connection lands 114 are connected to the IDT 112 for conducting electric power are formed by using the same material as that of the IDT 112. Therefore, the electrical connection can be established by performing wire-bonding to the connection lands 114.
To form a high-frequency-range and stable oscillator by using the surface acoustic wave device, a surface acoustic wave device having a large Q-value (sharpness of resonance) and a low resonance resistance and, therefore capable of resonating a stable resonance frequency, is required. A conventional surface acoustic wave device of the foregoing type comprises a surface acoustic wave resonator brought into close contact with a support substrate by an adhesive agent. Therefore, the difference in the coefficient of thermal expansion between the surface acoustic wave resonator and a portion for supporting the same, contraction of the adhesive agent, deformation of the supporting portion and the like cause the surface acoustic wave resonator to be distorted, as a result of which the resonance frequency to be made instable or the resonance resistance, is increased. Since many of these surface acoustic wave devices have been used as surface acoustic wave filters, considerably large Q-values have not been required. However, a surface acoustic wave resonator that forms an oscillator must be capable of generating resonance frequencies more stably than that required for the filter. Therefore, it is important to use a surface acoustic wave resonator unit having a low resonance resistance and a large Q-value in order to realize a stable oscillator.
In order to obtain a reliable oscillator, the device for forming the same must have excellent reliability. In a surface acoustic wave device, adhesion of foreign matter, such as dust, to the surface, on which the IDT has been formed, causes a problem to arise in that the frequency is changed or stable resonance characteristics cannot be obtained. If the connection between the IDT and the lead becomes defective, stable oscillation characteristics cannot, of course, be obtained, and an increase in the connection resistance occurring due to the defective connection causes the frequency to be changed and the Q-value to be lowered. Accordingly, it is also important to provide a reliable device by eliminating influence of foreign matter and preventing a defective state of connection while preventing influence of distortion on the surface of the surface acoustic wave resonator.